View Full Version : Pathlight spacing using footcandles
looking to add some pathlights to a walk at my house. My question is "visually" whats the lowest # of footcandles you want before you add another light. Example if I space 2 pathlights at 6' and each light has .5 footcandles, at the middle of the lights you would have close to 1 footcandle. Too much? Too Little? All the manufacturers spec the light w/ a foot candle chart but I can't visualize the light dropoff from the fixture.
05-25-2011, 12:12 AM
depends on specs of fixture (reflective light), can't just look at lamp. also depends on what kind of path you're trying to illuminate..smooth concrete(reflective)..stamped that is a dark color, absorbing light, stepping stones, aggregate.etc. are there safety issues? Is it a walkway the primary entrance your guests/friends/family will use? If so, have to consider how they will see coming and going. getting out of car, pupil dialated...exiting home, pupil constricted.
Thanks for the reply. Makes sense, but how can I use the photometric chart a basic starting point to design the lighting. I know each fixture has its own chart for different lamps and fixture heights, and that a concrete walk will reflect light differently than a water struck brick. Is there any "standards" or "basic rules of thumb" for those drop off points. I.E. main walk into building 1.5 fc , Side entrance 1.0 fc. It just seems to me if you know the illumination you are trying to achieve then you would pick the fixture, lamp and post height to get the desired effect. Anny recommendations for design resources?
If this is a commercial application than you will want to check with your State or city requirements. Here is a link to Maine's suggestions on proper footcandles for various applications. You can call or look on line for more information.
Most reputable manufacturers will provide you with specifications of light distribution of each of their products. You can then calculate the proper spacing required to acheive the proper amount of footcandles required by your State Laws or suggestions.
By ensuring that you meet those requirements you will have protected yourself from any liability.
A great resource to have is Jan Moyer's book "The Landscape Lighting Book" as she describes how to proceed with the proper calculations keeping in mind of the reflective surfaces in certain applications.
05-25-2011, 02:03 PM
Ken's advice is good.
Keep in mind that regional lighting ordinances never require a minimum illumination for a walkway on a residential property (though they might set restrictions on light trespass and light pollution).
IESNA document DG-5-94 (Recommended Lighting for Walkways and Class 1 Bikeways) does offer advice on lighting for residential paths. Similar information also appears in the IESNA Lighting Handbook, Jan Moyer's book, and others.
Note that these recommendations seem to be primarily aimed at walkways in multi-family residential properties and are divided (in DG-5-94) as High, Medium, and Low Usage areas.
They also specifiy ratios (avg. illuminance/min. illuminance) since a certain degree of uniformity is important.
Having said all that, my own experience is that an illumination of 0.05 fc is a good minimum to shoot for. Keep in mind that bright moonlight is about 0.01 fc. You may think that 0.05 is too low, but that's at the edges of the beams, the avg. illumination in the bright area of the beam would be closer to 0.5 fc. A 10:1 ratio (avg./min.) is good, even a 20:1 might be OK.
This would mean that, looking at an iso-footcandle chart, you would find the line on the chart for 0.05 fc and follow that to the beam radius. For example, if a certain path light indicates 0.05 fc at a radius of 4 ft., then you would space the fixtures 8 ft. apart to ensure a min. level of 0.05 fc.
Of course, in some situations (elderly, stairs, rough surface, etc.) you would want a higher illuminance level.
05-26-2011, 01:05 AM
I may be wrong, but I don't see too many footcandle charts for pathlights on the web. a thought that runs through my mind is they would have to be published for a certain voltage in LV applications, as a drop from 12v to 11v would represent a good 10 plus percent in lumen and footcandle drop. while a spacing by FC may make sense on paper, I find that purchasing 2 lights and an inexpensive trans, putting them in place and seeing what effects you get at different spacings on the very walking surface you will be using will be far more valuable to you. again, I would check the voltage at the fixtures, and plan accordingly. it is rare that that will be the exact voltage you get when you have 4-6 of them on a wire run, hub or no hub.
LED bulbs however, changes this equaton and levels the playing field a bit. they tend to put out consistant fc and lumens at 9v to 15v. might be worth a look.
05-26-2011, 11:10 AM
David, I agree, there's no replacement for seeing an in-field test, or using prior experience as a guide.
Regarding lumen drop for incandescents, a 10% drop in voltage (from 12V to 11V) results in a 30% loss of lumens and footcandles. (See GE document (http://www.gelighting.com/eu/resources/literature_library/product_brochures/downloads/halogen_reflector_en.pdf).)
This makes your case for on-site evaluation even more important. Iso-footcandle charts are only guides.
Thanks for clarifying and presenting the information so well. I always have trouble describing things so clearly on here sometimes.
I am sorry that I used the word "required" in the second sentence above. All of the information provided to us is only suggestions for footcandles but this does provide us with a starting point when designing in the office.
Chris captured important details too as surface conditions can vary the reflectance and light dispersement as well.
When proceeding to the mock up or installation your final night aiming is most crucial and David captured that in his post as well.
Lots of good information on how to space your pathlights.
Thanks everybody for sharing. I love this site for just that reason alone.
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